Three-dimensional Scour Below Submarine Pipelines Under Unidirectional Flow

Submarine pipeline is an important facility in offshore engineering and the lifeline for the transportation of offshore oil and gas resources.Its stability is an important issue of great concern to the engineering community.In practical engineering,submarine pipelines are often subject to local scour caused by hydrodynamic factors such as ocean currents,resulting in suspended spans and further fatigue damage of pipelines.Therefore,it is of great practical significance to fully understand the local scour mechanism of submarine pipelines and establish the prediction and analysis method of local scour propagation of submarine pipelines for the safety design,operation and maintenance of submarine pipelines and the saving of engineering costs.This paper focuses on the physical experiment and theoretical analysis of the three-dimensional scour propagation of submarine pipelines under unidirectional flow.Based on the local scour propagation experiment of submarine pipeline,the effects of physical factors such as flow direction,velocity and buried depth of pipeline on the scour propagation rate,the amplification factor of shear stress at the edge of scour hole and the slope angle of local scour hole are analyzed.Through the erosion test,the influence of sediment characteristic parameters on three-dimensional local scour is analyzed to solve the problem that the previous prediction model can not be applied to practical projects.On the basis of the experimental results of the two parts and the theoretical analysis,a formula to predict the scour propagation rate is established.The experiment results of this paper show that under the combined action of complex factors,the local scour propagation of submarine pipelines has two basic forms,namely,uniform scour propagation and non-uniform scour propagation.In the case of uniform scour propagation,the scour propagation rate remains constant.The amplification factor at the edge of the scour hole remains unchanged during the scour propagation.The scour profile under the pipeline axis changes continuously during the scour propagation,and the slope angle at the span shoulder remains unchanged during the whole scour process;In the case of non-uniform scour propagation,there is an extreme value of the suspended span length of the pipeline.The amplification factor at the edge of the scour hole decreases approximately linearly along the length of the pipeline span,and decreases to 1 at the end of the suspended span.The scour propagation rate also gradually decreases to 0 along the length of the pipeline span.Based on a large number of experiment results,this paper gives a method to judge the form of scour propagation.For the initial stage of local scour propagation,the dependence of the amplification factor of the bed shear stress on the depth-to-diameter ratio of the pipeline and the angle of attack of the incoming flow is established;Based on the theoretical analysis of scour propagation process,the evolution equation of scour profile under the pipeline axis with time is established,and the value of slope angle at the shoulder is given.Based on the comprehensive consideration and theoretical analysis of sediment characteristic parameters,amplification factor of shear stress and slope angle,an empirical model of scour propagation considering the influence of attack angle and depth-to-diameter ratio is established,which can predict the scour propagation rate.